Electronic devices, to be serviceable under a wide variety of environmental conditions, must be able to withstand moisture, heat and abrasion, among other stresses. A significant amount of work has been reported directed toward various protective measures to minimize the exposure of these devices to the above conditions and thereby increase their reliability and life. Most of these measures, however, suffer from various drawbacks.
For instance, early measures involved potting electronics within a polymeric resin and, thereby, reducing environmental exposure. These methods proved to be of limited value, however, since most resins are not impermeable to environmental moisture and generally add extra size and weight.
A second method of protection involves sealing the device within a ceramic package. This process has proven to be relatively effective in increasing device reliability and is currently used in select applications. The added size, weight and cost involved in this method, however, inhibits widespread application in the electronic industry.
Recently, the use of lightweight ceramic coatings has been suggested. For instance, Haluska et al. in U.S. Pat. No. 4,756,977, which is incorporated herein in its entirety by reference, discloses silica coatings produced by applying solutions of hydrogen silsesquioxane to an electronic device and then ceramifying by heating to temperatures of 200.degree.-1000.degree. C. This reference also describes the application of other coatings containing silicon carbide, silicon nitride or silicon carbonitride onto the initial silica layer for added protection. The ceramic coatings produced thereby have many desirable characteristics such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength and thermal expansion coefficient matching which provide excellent protection to the underlying substrate.
Haluska et al. in U.S. Pat. No. 4,753,855 also proposed the application of solutions comprising hydrogen silsesquioxane resin (H-resin) and metal oxide precursors of titanium, zirconium and aluminum to a substrate and pyrolyzing said substrate to temperatures of 200.degree.-1000.degree. C. to form a mixed oxide ceramic coating. This patent, however, fails to teach or suggest the use of metal oxide precursors other than those of titanium, zirconium and aluminum. One skilled in the art would not be able to select other metal oxide precursors based on this disclosure, especially since it is known that the inclusion of certain oxide precursors may be detrimental. For instance, it is known that certain metal oxide precursors may 1) cause gelation of the preceramic mixture (iron oxides, for example): 2) react with the substrate to cause degradation of the preceramic mixture or the substrate (sodium or potassium oxides, for example): or 3) induce stress on the substrate when the oxide precursor expands or contracts upon heating (see, for example, the discussion of Dietz et al infra, column 1, lines 28-62).
Japanese Kokai Patent 63289939 discloses the formation of coating films by hydrolyzing an alkoxysilane or a halogenated silane in a solvent to form a preceramic mixture, applying the preceramic mixture to a substrate and heating at 300.degree. C. or below in an ozone environment. Various glass forming agents including the oxides of boron, phosphorous and tantalum may also be included in the preceramic mixture (page 8, lines 20-23). This reference, however, does not describe the use of hydridosilanes in formation of the preceramic mixture. As such, none of the compounds listed therein could be hydrolyzed and condensed to form H-resin. Moreover, since one skilled in the art readily recognizes the instability of the Si-H bond, it would be expected that materials containing said Si-H bond (such as H-resin) may have different properties (such as solubility and stability) than materials containing the hydrolysis products of the compounds disclosed in the reference.
Dietz et al. in U.S. Pat. No. 3,858,126 teach the formation of a composition comprising 15-60% PbO, 12-40% B.sub.2 O.sub.3 and 5-45% ZnO, with optionally up to 12% other oxides such as SiO.sub.2. This composition is applied to a substrate and pyrolyzed to a temperature less than about 650.degree. C. to form a hermetic coating on said substrate. This reference, however, teaches that the oxides of Pb, B and Zn must be present to be effective and, if SiO.sub.2 is included, it must be present in an amount less than 12%.
Levene et al. in U.S. Pat. No. 3,640,093 describe a process of preparing a high purity oxide comprising partially hydrolyzing a silicon alkoxide, reacting the partial hydrolysate with a metal alkoxide and adding a sufficient quantity of water to form a gel. This reference does disclose that the silicon alkoxide may be substituted with hydrogen (Column 1, line 50) and that metal alkoxides similar to those disclosed herein may be utilized (column 1, line 62). This reference, however, (1) discloses that the resultant product is a gel, (2) does not teach coating electronic devices and (3) only provides Examples which show the incorporation of a few metal alkoxides with silicon compounds substituted with 4 functional groups.
Levene in U.S. Pat. No. 3,811,918 teaches the formation of a gel resistant glass precursor composition which may be heated to form a protective glass coating. The coating composition is formed by a method comprising partially hydrolyzing silicon alkoxides (including H-substituted), reacting the partial hydrolysate with an aqueous solution of a metal oxide forming compound, hydrolyzing this solution with additional water and then adding an acid to form a stable, gel-free solution. This reference, however, fails to provide examples wherein a hydridosilicon compound is utilized or examples wherein the multitude of metal oxide precursors disclosed therein are utilized. This reference also requires acid stabilization for stable glass precursor solutions, the application of the preceramic solution to a hot substrate (column 7, lines 7-17), and the use of temperatures above 1000.degree. C. to form the glass coating (see the examples).
The present inventor has unexpectedly found that the oxide precursors of tantalum, vanadium, niobium, boron and/or phosphorous can be mixed with hydrogen silsesquioxane resin to form a soluble preceramic mixture which can be applied to a substrate and pyrolyzed to yield a ceramic or ceramic-like coating on said substrate.